This invention relates to an acceleration sensor and, more particularly, to an acceleration sensor suitable for detecting acceleration based on earthquake or collision of automotive vehicle.
In view of grasping movement of an object, detection of acceleration has important meaning. For this reason, various acceleration sensors have been conventionally proposed. Particularly, in recent years, the spotlight of attention upon multi-dimensional acceleration sensors capable of detecting two-dimensional or three-dimensional acceleration every respective directional components has been focused. For example, in the International Publication No. WO88/08522 based on the Patent Cooperation Treaty, a three-dimensional acceleration sensor using piezo resistance element is disclosed. In this sensor, plural piezo resistance elements are formed at specific positions on a semiconductor substrate, thereby making it possible to respectively independently detect acceleration components in respective coordinate axes directions in the XYZ three-dimensional coordinate system. Moreover, in the International Publication No. WO91/10118 or the International Publication No. WO92/17759, a three-dimensional acceleration sensor using electrostatic capacitance elements is disclosed. In the International Publication No. WO93/02342, a three-dimensional acceleration sensor using piezoelectric elements is disclosed. In these sensors, plural electrodes are formed at specific positions, thereby making it possible to respectively independently detect acceleration components in respective coordinate axes directions in the XYZ three-dimensional coordinate system in a manner similar to the above.
In such three-dimensional acceleration sensors, since all of respective coordinate axes direction components of applied acceleration can be respectively independently detected by a single sensor, it is possible to specify acceleration to be detected as a vector quantity within the three-dimensional space. Accordingly, such three-dimensional acceleration sensors can be widely utilized in use for precisely detecting acceleration exerted at an object moving within the three-dimensional space, e.g., a cruising vehicle, or an air-plane in flight, etc. in a manner to include its direction. In future, it is expected that its utilization value will be increased.
On the other hand, the acceleration sensor can be utilized also as a seismometer or an impact meter. For example, in a control system for controlling valves of city gas infrastructure or a control system for elevators, an acceleration sensor functioning as a seismometer is included. When an acceleration based on vibration of earthquake exceeds a predetermined threshold value, the control system operates to stop supplying of gas, or to stop operation of the elevator. Moreover, in automotive vehicles with an air bag system which becomes popularized rapidly in recent years, an acceleration sensor functioning as an impact meter is mounted. In this system, there is employed a mechanism to momentarily swell the air bag to protect a driver in the case where an acceleration based on impact exceeds a predetermined threshold value. However, the conventional seismometer or the impact meter generally includes a mechanical sensor instead of the above-described three-dimensional sensors. The mechanical sensor has a function to mechanically detect an acceleration by such a way to recognize whether or not a steel ball is flown out from a bowl-shaped vessel.
As described above, in the conventional seismometer or impact meter, mechanical acceleration sensors are mainly utilized. However, in such a mechanical acceleration sensor, there are problems that detection accuracy or reliability is low and that it is difficult to electrically take out detection result. On the other hand, three-dimensional acceleration sensors using piezo resistance elements, capacitance elements or piezoelectric elements have high detection accuracy and reliability, and can electrically take out detection results. However, in the purpose for the seismometer or the impact meter, such three-dimensional acceleration sensors are not necessarily required and there are even instances where such three-dimensional acceleration sensors are not suitable.
For example, in the purpose for measuring intensity of earthquake, it is sufficient to provide a function to respectively independently detect the so-called "transverse vibration (vibration in the horizontal direction)" and the so-called "longitudinal vibration (vibration in the vertical direction)" by the earthquake. At this time, it is desirable to have an ability of directly detecting the magnitude of "transverse vibration" and the magnitude of "longitudinal vibration". Generally, it is known that "transverse vibration" in the earthquake is vibration resulting from vibrating wave called "S wave", and the "longitudinal vibration" is vibration resulting from vibrating wave called "P wave". As long as it is possible to respectively independently detect the magnitude of the S wave and the magnitude of the P wave, such detection mechanism can sufficiently function as a seismometer. Namely, when an XYZ three-dimensional coordinate system respectively having an XY-plane on the horizontal surface and a Z-axis in the vertical direction is defined, if the magnitude of vibration in the direction along the XY-plane (transverse vibration) and the magnitude of vibration in the direction along the Z-axis (longitudinal vibration) can be measured, such detection mechanism can sufficiently satisfy the seismometer.
It is a matter of course that even if the conventionally proposed three-dimensional acceleration sensor is employed, the above described measurement can be made. When the conventional three-dimensional acceleration sensor is used for a seismometer, precise detection in which even the azimuth is specified such as "transverse vibration along the direction of the north-northeast" can be made. However, for the purpose of carrying out supply control of the city gas or operation control of the elevator, it is unnecessary to specify an azimuth of the transverse vibration. When the magnitude of transverse vibration exceeds a predetermined threshold value, irrespective of "transverse vibration along the direction of the north-northeast" or "transverse vibration along the direction of southeast", it is necessary to stop supply of the city gas or to stop operation of the elevator. In other words, as long as the magnitude of the transverse vibration can be detected, it is possible to sufficiently perform the function as an acceleration sensor used in the seismometer. Moreover, in the conventional three-dimensional acceleration sensor, since X-axis direction component .alpha.x, Y-axis direction component .alpha.y and Z-axis direction component .alpha.z are respectively independently detected with respect to the acceleration in the XYZ three-dimensional coordinate system, a mathematical operations (calculations) to obtain a sum of .alpha.x.sup.2 +.alpha.y.sup.2 and a square root of this sum are required in order to determine the magnitude of transverse vibration along the XY plane.
As stated above, the conventional three-dimensional acceleration sensor can be used as a seismometer. However, since the structure becomes complicated and a specific operation circuit to be adopted for a seismometer is required, there results the problem that the cost is increased as a whole. Particularly, when attention is drawn to utilization to the supply control of the city gas or the operation control of the elevator, it is necessary to provide such acceleration sensors within respective gas meters which are installed in respective homes or within respective control units which are installed in respective elevators. Therefore, a low cost acceleration sensor having a simple structure is expected.
Such circumstances are the same also in acceleration sensors used as an impact meter for operating the air bag system of the automotive vehicle. If the cruising surface of the automotive vehicle is assumed to be an XY plane, impact produced by collision of the vehicle is the impact mainly including an acceleration component along the XY plane. Accordingly, the acceleration component along the Z-axis can be neglected. In addition, irrespective of whether corresponding collision is the frontal (head-on) collision or side collision, it is the common fact that the impact to make a driver injured is to be applied. Namely, even if collision in any direction takes place, there is the necessity of swelling the air bag to protect the driver. Accordingly, if magnitude of acceleration component in a direction along the XY plane can be detected, such acceleration detection sufficiently satisfies the purpose. Namely, it is unnecessary to precisely detect the direction of the acceleration.